In many systems which have sensor networks for sensing system operating parameters, filters are used in the sensor networks in order to optimise the operation of the network: for example, to ensure good noise immunity and to limit the voltage supplied from the sensor. The latter is particularly important for tachometer (tacho) sensors in automotive applications. Typically, the time constants of the inductive and capacitive sensors of the networks are low compared to that of the filters. For these applications therefore, the sensors require close monitoring in order to ensure their proper operation.
By monitoring the DC current flowing through the sensor, it is possible to determine whether the sensor is tied to ground, tied to the power supply or whether the sensor is disconnected. However, this type of monitoring does not allow for the detection of a short-circuited sensor due to the low internal inductance of the sensor compared to the other components in the network.
One method of detecting sensor short-circuit is to apply a step or Dirac pulse to the network and observe the closed loop system response. However for tachometer sensors, such a method does not accurately or reliably discriminate between the short-circuited or not short-circuited sensor response. Furthermore, the time constant of the tachometer sensor, when combined with the passive filter components, becomes insignificant compared to the other time constants of the network.